1. Field of the Invention
The present invention relates to an image processing method, image processing apparatus, and program that can increase visual black density and improve tone reproducibility.
2. Description of the Related Art
Today, image output apparatuses, such as color printers and copiers, are expected to provide high print quality. Among the requirements for achieving such high print quality are reduced granularity of printed images, improved color matching with monitor colors and with memory colors, improved tone reproducibility, and increased black density. A number of image processing techniques have been proposed to meet these requirements.
Of the requirements described above, black density exhibits variations that are particularly significant in low-luminance images, such as in nightscape images. For example, low black densities produce faded and blurry images. Therefore, higher black densities are required to prevent blurry images from being produced.
Densities, including black densities, can be measured by the spectral characteristics of reflected light. However, the spectral characteristics are variously affected by the characteristics of recording media. The examples include the color, smoothness, and oil absorbency of the recording media, and the characteristics of their coating material, such as fluorescent material. Japanese Patent Laid-Open No. 2002-223362 discloses a method in which a gray scale is controlled by adjusting a hue angle so that a desired gray level can be reproduced regardless of the types of recording media. That is, what is disclosed is a method for achieving target black densities and natural grayscale images. Moreover, Japanese Patent Laid-Open No. 2002-232727 discloses a method in which a visual check allows the correction of a shift in gray balance or in color that occurs at the time of the replacement of recording paper.
The method disclosed in Japanese Patent Laid-Open No. 2002-223362 adjusts gray levels based on the assumption that optically measured data is equivalent to the current gray level. However, a gray level obtained by this adjustment may differ from that visually obtained. This is because when actual samples are evaluated, black densities determined by measurements differ from those determined by visual checks. Black densities that are actually high may not be able to be obtained, in particular, by visual checks.
Although Japanese Patent Laid-Open No. 2002-232727 discloses a method in which a visual check allows the correction of a shift in gray balance, there is no suggestion in this document for achieving higher visual black densities.